MOMENTUM by Brent Angeline

A dying Earth demands exploration of the galaxy in cryogenic ships traveling at time-dilated relativistic speeds in an effort to find resources... and, ultimately, a new home planet. A timid techie's nanobots fail to mine in an asteroid belt, and the hundred-year grand-scale mining operation that follows fails as well. However, first contact with a mysterious alien species reveals the locations of resources and even new home worlds for humans, but it also reveals war with an aggressively hostile alien species coming for all life in the galaxy and beyond. Humanity's first battles in the conflict expose that the war is a losing one, but the mysterious alien allies have one final assignment to give. In a last-ditch effort, the nanobot technician and a team of humans mutated and evolved by cosmic radiation must race to find and activate a strange and incredible alien device to save the universe -- but at what cost?